A central objective of the refinement of packet networks is currently to be able to offer real-time services via packet networks. The development tasks include switching, networking and Internet technology aspects.
Packet networks are conventionally used primarily for transmitting non-time-critical data such as file transfer or electronic mail. The aim of more recent developments is also to transmit services with real-time requirements such as telephony, which is conventionally handled via circuit-switched networks, and video information.
This intended expansion of the service spectrum involves corresponding requirements on transmission quality. For services with real-time transmission, specific quality parameters such as limits for data packet delay and loss must be complied with. In this context, the technical literature frequently refers to Quality of Service, abbreviated to QoS, parameters or requirements.
The most widely used packet network technology, which is based on IP (Internet Protocol), conventionally provides only best effort routing for the transmission of packets. Packets are transmitted from point to point or from router to router, the relevant router only deciding locally about the onward transmission of the packets to the next router. Internet terminology refers to individual hops. Within the framework of said best effort method, no quality of service parameters can be guaranteed. No corresponding monitoring of the delay times or packet loss rates is provided. These two parameters may assume considerable values, e.g. when bottlenecks occur in the case of individual routers or on individual links.
One approach to ensuring the monitoring necessary for quality of service level transmission in packet networks is to reserve paths from end to end or from host to host. Methods employing path reservation include ATM (Asynchronous Transfer Mode) or for the IP networks MPLS (Multi Protocol Label Switching). Reserving complete paths has the disadvantage of high complexity, low flexibility and poor resource utilization. Other methods have therefore been developed for the IP network, which at least in part preserve the flexibility of the original IP concept using best effort. One such approach is the Diff Serve (Differentiated Services) concept whereby packets are prioritized on entry to a subnetwork and are handled within said subnetwork according to their prioritization. Ultimately, however, this approach does not lead to genuine quality of service guarantees but only to the introduction of class-of-service categories.